Beats

When two sounds with nearly the same frequency play together, they drift in and out of step with each other. Sometimes their vibrations add up to make a louder sound, and sometimes they cancel out to make it quieter, so the loudness pulses up and down. This pulsing rate, called the beat frequency, is simply the difference between the two frequencies.

f_beat = |f₁ − f₂|

• f_beat — beat frequency (Hz): how many times per second the loudness pulses
• f₁ — frequency one (Hz): the frequency of the first sound
• f₂ — frequency two (Hz): the frequency of the second sound

A tuning fork rings at 442 Hz, and a guitar string rings at 438 Hz. How many beats per second will you hear?

• f₁ = 442 Hz
• f₂ = 438 Hz

1. f_beat = |f₁ − f₂|
2. f_beat = |442 Hz − 438 Hz|

f_beat = 4 Hz

A guitar string rings at 440 Hz, and a tuning fork rings at 445 Hz. How many times per second will the sound pulse louder and quieter?

• 5 times per second
• 885 times per second
• 440 times per second

You're tuning a guitar string to match a 440 Hz reference. As you tighten the string closer to the correct pitch, what happens to the beats you hear?

• They speed up
• They slow down and eventually disappear
• They stay the same speed